This application claims priority under 35 USC xc2xa7119 of German Application Number 19919234.0 filed on Apr. 28, 1999.
The invention relates to a method and apparatus for coating strip-shaped material with covering material, in which the strip-shaped material which is to be coated is guided into the coating bath, between two coating rolls, which are partly immersed in the coating bath, out of the coating bath and past stripping nozzles; the aforesaid components being collectively referred to as a coating plant.
Coating plants for coating strip-shaped material have been known in diverse forms for a long time. The purpose of such a plant is to cover strip-shaped material of different types, i.e. metal strip, plastics material strip, fabric strip or paper strip, with different coverings of liquid media, e.g. with molten zinc, tin or alloys thereof or colouring material. The coating bath for coating the strip-shaped material with tin or zinc is in the form of a melted bath in which the materials are melted.
Coating plants of the type initially mentioned have been known for a long time, e.g. from JP-A 55128570. This coating plant is provided with a deflector roll which guides the strip through the bath and which is completely immersed with its bearings in the coating bath. Also provided are two coating rolls which are partly immersed in the coating bath and the bearings of which are likewise immersed in the latter. The strip-shaped material is guided around the deflector roll and then routed through the gap between the two coating rolls to the stripping nozzles. The deflector roll is arranged below the coating rolls in the coating bath in this coating plant such that the strip-shaped material passes upwardly through the gap between the two coating rolls. The strip is therefore firstly brought by means of the deflector roll into a position from which it passes from the bottom upwards through the gap between the two coating rolls.
As the bearings of the coating rolls and also of the deflector roll are immersed in the coating bath in this coating plant, they are subject to high wear levels on account not just of high friction- and flow-related stress, but also thermal stress, and have to be replaced frequently. However their replacement always entails bringing the complete line to a standstill and a considerable expenditure of time, as the rolls must be lifted out. This results in an interruption of the continuous operation andxe2x80x94following replacementxe2x80x94numerous working steps until the plant is returned to its working state.
Moreover, the strip which is to be coated cannot travel quickly without limitations in this known plant. An excessive amount of material is dragged out of the bath particularly at relatively high speeds, as the coating rolls are arranged at a certain distance from one another. A greater amount of material must accordingly be stripped off by means of the stripping nozzles.
In order to prevent splash phenomena in the case of coating baths where the strip is travelling at a fairly high speed, it is known from JP-A 55085664 to arrange the guide rolls, through which the strip which is to be coated travels on or under the surface of the coating bath, so that the distance between the surface of the coating bath and the contact point between the strip which is to be coated and a roll lies within the diameter of the roll. This known plant also provides regulation of the contact pressure force between the rolls and the intervening strip which is to be coated, with the possibility of uncurving the latter. However, in this known plant, in addition to the deflector roll, at least one guide roll is immersed in the coating bath such that its bearings are located in the latter. This arrangement therefore also entails the above-mentioned wear phenomena for the bearings with the necessity of replacing the corresponding rolls while shutting down the plant. In contrast, an object of the invention is to provide a coating plant of the type initially mentioned whose mobile parts have an extended life and to increase the speed of the strip which is to be coated.
This object is solved by arranging the bearings of the coating rolls outside the coating bath.
With this type of structure of the coating plant, in which the bearings of the coating rolls lie outside of the coating bath, the bearings of these rolls are only subject to normal wear. Moreover, the rolls and their bearings can be accessed far more easily from outside, so that only a minimum amount of time is required to carry out a replacement if a bearing or a roll becomes defective. As the bearings of the coating rolls and therefore also their axles are located outside the coating bath, very little coating material is entrained out of the coating bath, as the space between the coating bath surface and the roll surface is firstly filled with coating material and only the adherent material is available for coating, without the presence of any further material above the rolls. The claimed coating plant enables higher strip speeds to be achieved. The coating material also exhibits no meniscus at the strip, as any coating material which is present in excess above the rolls is conveyed away from the strip by the rolls.
Advantages are obtained in particular when employing the galvannealing process on account of the short periods of immersion in the coating bath.
The rolls may be provided with a special coating in order to increase the service life of the coating rolls even when the bearings are mounted outside of the coating bath.
The maintenance costs will also be substantially lower than in the case of previous plant because of the above-mentioned advantages.
Advantageous developments are characterised by the subclaims.
Advantages are obtained with regard to the service life of the deflector roll bearings in a coating plant of the described type, in which the strip-shaped material is guided into the coating plant by means of one or more deflector rolls, if the bearings of one deflector roll or all deflector rolls are arranged outside of the coating bath. This arrangement of the deflector roll bearings enables the coating material container holding the coating bath to be substantially shallower than in the case of previous coating plant, in which one deflector roll is always located below the coating rolls in the coating bath. This naturally applies to a plant structure which is without any deflector rolls for the strip-shaped material which is to be coated, with the latter being fed directly to the coating rolls and deflected by one of these.
Just one deflector roll may be provided, this being located outside of the coating bath. In this case this roll works with minimum friction losses. If the deflector roll is located outside of the coating bath or if no deflector roll is provided, the coating bath container may also be particularly small.
The coating rolls can advantageously be displaced horizontally and vertically, so that the plant can be adapted to different thicknesses of strip to be coated.
Differential coatings can be achieved if the coating rolls are adjusted in distance from the strip shaped material.
The stripping nozzles are preferably electrically heated. They can therefore easily be regulated. The use of a hood effectively prevents oxidation of the coating, especially in the case of metal coatings, due to a closed atmosphere, e.g. due to N2. The viscosity of the coating can be maintained under a hood, so that, together with high stripping medium temperatures, excess coating material can be reliably stripped off. The speed of the strip which is to be coated can also be directly increased with high coating material viscosities. Temperatures of up to 600xc2x0 C. may be employed for the hot stripping of metallic materials. This results in significant advantages in the galvannealing process.
Coating rolls of different diameters may be used, with greater roll diameters being selected for high speeds to prevent the coating medium from splashing due to centrifugal forces. Variable strip speeds of 30 to at least 300 metres per minute can be achieved.
Not only the coating rolls, but also the deflector roll if it is immersed in the coating bath, are preferably provided with strippers in order to strip off accumulating slag.
The strip-shaped material may be metal strip, plastics material strip, fabric strip or paper strip. Different coating baths with coating temperatures of appropriate settings are then used accordingly. The coating material used may be, e.g. zinc, aluminium, tin and alloys of a wide variety of types, and it is possible to use, for example, both liquids and colouring powder dissolved in water.